I had same issue and below is resolved.

Old Service Code:

@Injectable()

Updated working Service Code:

@Injectable({
  providedIn: 'root'
})

Open: ./src/app/app.module.ts
And import Firebase Modules at the top:

import { environment } from '../environments/environment';
import { AngularFireModule } from 'angularfire2';
import { AngularFirestoreModule } from 'angularfire2/firestore';

And VERY IMPORTANT:
Remember to update 'imports' in NgModule:

@NgModule({
  declarations: [
    AppComponent,
    OtherComponent // Add other components here
    ...
  ],
  imports: [
    BrowserModule,
    AngularFireModule.initializeApp(environment.firebase, 'your-APP-name-here'),
    AngularFirestoreModule
  ],
  ...
})

Give it a try, it shall now work.
For detailed information follow the angularfire2 documentation:

Good luck!

Simple Answer.

Whenever you encounter NullInjector error. Reason is always import/providers is missing.

SOLUTION:

1. Please add the module in the imports array inside your module.ts file.

2. Add the services you want to use in the providers array inside your modules.ts file.

@NgModule({})
export class SomeModule{
 `imports:[]`,
`providers: []`
}

Weird thing for me was that I had the provider:[], but the HTML tag that uses the provider was what was causing the error. I'm referring to the red box below:

It turns out I had two classes in different components with the same "employee-list.component.ts" filename and so the project compiled fine, but the references were all messed up.

Adding AngularFirestoreModule.enablePersistence() in import section resolved my issue:

imports: [
    BrowserModule, AngularFireModule, 
    AngularFireModule.initializeApp(config),
    AngularFirestoreModule.enablePersistence()
]

I solved this problem by just removing firestore from:

import { AngularFirestore } from '@angular/fire/firestore/firestore';

in my component.ts file. as use only:

import { AngularFirestore } from '@angular/fire/firestore';

this can be also your problem.

For AngularFire2 Latest version

Install AngularFire2

$ npm install --save firebase @angular/fire

Then update app.module.ts file

    import { BrowserModule } from '@angular/platform-browser';
    import { NgModule } from '@angular/core';

    import { AppComponent } from './app.component';
    import { FormsModule } from '@angular/forms';


    import { AngularFireModule } from '@angular/fire';
    import { AngularFireDatabaseModule } from '@angular/fire/database';
    import { environment } from '../environments/environment';
    import { AngularFirestoreModule } from '@angular/fire/firestore';


    @NgModule({
      declarations: [
        AppComponent
      ],
      imports: [
        BrowserModule,
        FormsModule,

        AngularFireModule.initializeApp(environment.firebase),
        AngularFirestoreModule,
        AngularFireDatabaseModule
      ],
      providers: [],
      bootstrap: [AppComponent]
    })
    export class AppModule { }

Check FireStore CRUD operation tutorial here

Change Your Import From :

import { AngularFirestore } from '@angular/fire/firestore/firestore';

To This :

import { AngularFirestore } from '@angular/fire/firestore';

This solve my problem.

I take that to my app.module. After the imports it should be works

providers: [
    { provide: LocationStrategy, useClass: HashLocationStrategy },
    { provide: FirestoreSettingsToken, useValue: {} }
  ],

My Version:

Angular CLI: 7.2.4
Node: 10.15.0
Angular: 7.2.5
... common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.12.4
@angular-devkit/build-angular     0.12.4
@angular-devkit/build-optimizer   0.12.4
@angular-devkit/build-webpack     0.12.4
@angular-devkit/core              7.2.4
@angular-devkit/schematics        7.2.4
@angular/animations               8.0.0-beta.4+7.sha-3c7ce82
@angular/cdk                      7.3.2-3ae6eb2
@angular/cli                      7.2.4
@angular/fire                     5.1.1
@angular/flex-layout              7.0.0-beta.23
@angular/material                 7.3.2-3ae6eb2
@ngtools/webpack                  7.2.4
@schematics/angular               7.2.4
@schematics/update                0.12.4
rxjs                              6.3.3
typescript                        3.2.4
webpack                           4.28.4

import angularFirebaseStore 

in app.module.ts and set it as a provider like service

I had the same issue while adding firebase to my Ionic App. To fix the issue I followed these steps:

npm install @angular/fire firebase --save

In my app/app.module.ts:

...
import { AngularFireModule } from '@angular/fire';
import { environment } from '../environments/environment';
import { AngularFirestoreModule, SETTINGS } from '@angular/fire/firestore';

@NgModule({
  declarations: [AppComponent],
  entryComponents: [],
  imports: [
    BrowserModule, 
    AppRoutingModule,
    AngularFireModule.initializeApp(environment.firebase),
    AngularFirestoreModule
  ],
  providers: [
    { provide: SETTINGS, useValue: {} }
  ],
  bootstrap: [AppComponent]
})

Previously we used FirestoreSettingsToken instead of SETTINGS. But that bug got resolved, now we use SETTINGS. (link)

In my app/services/myService.ts I imported as:

import { AngularFirestore } from "@angular/fire/firestore";

For some reason vscode was importing it as "@angular/fire/firestore/firestore";I After changing it for "@angular/fire/firestore"; the issue got resolved!